How to zoom out of the backdrop in the compositor?
The four controls for manipulating a Background Image in the Compositor:


V zooms out.
Alt + V zooms in.
Alt +  MMB pans around.
Alt + Home fits backdrop to the available area.


Also note the changes to the Node Editor Properties panel, such as the addition of the Backdrop dropdown, which, among other things like the hotkeys, allows for precision zooming.



Refer to the Manual for additional info.